This invention is directed to an improved process for preparing polyarylates which are melt stable and low in color. Most polyarylates range in color from straw to amber. Modern Plastics Encyclopedia, 1989. As is shown in the literature, it is difficult in commercial production to achieve low color polyarylates. The reason for the difficulty and the solution is fully discussed infra.
Polyarylates are polyesters derived from a dihydric phenol and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative, which is then reacted with an aromatic dicarboxylic acid(s), to form the polyarylate.
U.S. Pat. No. 4,075,173 issued Feb. 21, 1978, describes the preparation of copolyesters by reacting an aromatic dicarboxylic acid, a diacetate of Bisphenol-A, and an acetate of p-hydroxybenzoic acid. Various processes for producing polyarylates by the reaction of Bis-phenol-A and terephthalic and isophthalic acids are reviewed in this patent. The following process for producing polyarylates, identified as route (1), is described in column 2, of the patent: ##STR1##
This process is the diacetate process as described herein, or the "Acetate Process" as defined in the patent.
Column 2 of the patent states:
"The route (1) is not desirable because the undesirable coloration and deterioration of polymer are particularly remarkable as disclosed in the above-mentioned literature."
Further, column 3 of the patent states:
"On the other hand, the route (1), Acetate process, is economically advantageous because the materials used are cheap and the operation is simple. For example, diacetate of bisphenol-A, a monomer for Acetate process, is synthesized by merely reacting acetic anhydride and bisphenol-A. Consequently, it may be said that, if the fatal drawbacks of Acetate process, color and deterioration, are solved, Acetate process will become the most superior process."
Thus, the skilled workers in the field of polyarylate chemistry realize that the existing processes for producing polyarylates have one or more deficiencies, and that a need exists to develop a viable diacetate process for producing polyarylates.
In the following U.S. Patents, novel methods for producing polyarylates by the diacetate process are described.
U.S. Pat. No. 4,294,956, filed Aug. 27, 1979 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether" describes a process for preparing polyarylates having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm, the process comprises reacting at least one diester derivative of a dihydric phenol with at least one aromatic dicarboxylic acid in the presence of a diphenyl ether compound, at a temperature of from about 260.degree. to about 350.degree. C.
U.S. Pat. No. 4,294,957, filed Aug. 27, 1979 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates of improved color which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260.degree. to about 350.degree. C. Optionally, the process may be carried out in the presence of a magnesium, manganese, or zinc catalyst.
U.S. Pat. No. 4,296,232, filed Aug. 27, 1979 in the name of L. M. Maresca, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether Compound and A Catalyst" describes a process for preparing polyarylates which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of a diphenyl ether compound at a temperature of from about 260.degree. to about 350.degree. C. and in the presence of a magnesium catalyst.
However, in the U.S. Patents discussed above, the polyarylates produced by the diacetate process described therein still tend to contain colored species to an unacceptable extent and tend to be melt unstable if the intermediate dihydric phenol diester is not carefully purified prior to polymerization. Thus, the polyarylate must be prepared from a highly purified intermediate dihydric phenol diester, or it is difficult to fabricate. Also without purification of the diester, the polyarylate may not be acceptable in applications where polyarylates which are low in color are required.
U.S. Pat. No. 4,321,355 issued Mar. 23, 1982 to Maresca, et al, describes an improved process for preparing a polyarylate via the diacetate process. The improvement comprises removal of residual acid anhydride to less than 1500 parts per million after formation of the dihydric phenol diester. The patent suggests removal of the residual acid anhydride by vacuum distillation, or by chemical reactants which are not harmful to the polymerization such as water, alcohols, dihydroxy compounds and the like. Further, the patent examples describe removal of both acid anhydride and acetic acid by vacuum distillation or the addition of Bisphenol A.
While vacuum distillation is effective for small batches, it is difficult to consistently obtain these results on large batches due to the need for high vacuum, which cannot be routinely achieved or which requires expensive equipment. It has now been discovered that the addition of a C.sub.2 -C.sub.8 aliphatic monocarboxylic acid, preferably glacial acetic acid, achieves the necessary reduction of acid anhydride, and achieves a lower color polymer. As above stated, while it is possible in the laboratory to lower the residual acid anhydride concentration low enough to produce a stable, low color polymer by vacuum distillation alone, it has produced inconsistent results in production. Acetic anhydride levels varied from 1520 ppm to 13,000 ppm in production batches.
Various acids have been used in the prior art such as in esterification reactions as catalysts, but not in the context as shown herein. For example, in an article by Jose Erdos, et al. titled "Esterification of Phenol With Organic Anhydrides Using Chlorosulfonic Acid Catalysts" in Anales De La Escuela Nacional De Ciencias Biologicas, Vol. VIII, the esterification of phenols with organic anhydrides using acid catalysts is described. Specifically, the reaction of phenol with acetic anhydride in the presence of chlorosulfonic acid as catalyst is described. The catalyst is used in amounts of from 0.001 to 0.1 moles. The best yield of ester is stated to be when 0.001 mole of chlorosulfonic acid is used. This amount of the acid is equivalent to about 51 moles per million grams of reaction mixture.
The article further describes the use of concentrated sulfuric acid as a catalyst for the reaction of phenol and acetic anhydride. The sulfuric acid is used in amounts of from 0.0005 to 0.002 moles. The best yield (77.53%) of ester occurs when at least 0.001 mole of the acid is used. 0.001 mole of sulfuric acid in this system is equivalent to about 51 moles per million grams of reaction mixture.
In an article titled, "Acetic Anhydride-Phosphoric Acid as an Acetylating Agent", Carbohyd. Res., 6 (1968), pages 237-240 there is described that an acetic anhydride-phosphoric acid reagent is effective for acetylating carbohydrates, cyclitols, enols, phenols, etc. to produce acetates. The acetylating agent is used in amounts of from 10 to 25 milliliters to provide acetate in yields of from 55 to 92 percent.
M. V. Nekhoroshev, et al. in a publication titled, "Methods of Acylation of Sterically Hindered Phenols", Zhurnal Organicheskoi Khimii, Vol. 13, No. 3, page 662, March, 1977, describes the acylation of a phenol. Specifically, acetic anhydride is reacted with 4-alkyl-2,6-di-tert-butylphenol in the presence of perchloric acid as catalyst to yield (85-90 percent) the acetate. 1-2 drops of the perchloric acid catalyst are used. This is equivalent to about 200 moles per million grams of reaction mixture.
Additionally, in an article by M. Levine, et al. titled, "Properties of Polyesters of Bisphenols and Dicarboxylic Acids", Journal of Polymer Science, Vol. XXVIII, 5 1958, pages 179-184, the acetylation of bisphenol A is described. Specifically, bisphenol A is heated with acetic anhydride and 8 drops of sulfuric acid as catalyst to form the ester. The 8 drops of acid are equal to about 0.0075 moles which is equivalent to about 88 moles per million grams of reaction mixture.
It can thus be seen that the acylation of hydroxy-containing compounds with anhydrides has been catalyzed with large amounts of acid catalysts. The use of such large amounts of strong acids leads to handling problems as well as corrosion in the equipment used to prepare the ester.
It has now been unexpectedly found that residual acid anhydride can be most efficiently removed from the crude dihydric phenol diester reaction product by vacuum distillation followed by the addition of acetic acid, preferably glacial acetic acid (in order to minimize introduction of water), and followed by vacuum distillation. Since acetic acid is a byproduct of the reaction, it is not an additional contaminant and therefore does not require additional purification, and results in a polymer that is melt stable, and low in color. The residual acid anhydride is removed by vacuum distillation created by exerting 15 mm Hg pressure or less, then the addition of C.sub.2 -C.sub.8 aliphatic monocarboxylic acid, preferably glacial acetic acid, to the monomer reaction and distilling acid anhydride and acetic acid so that the acid anhydride concentration is less than about 1500 parts per million. It has also been found that the addition of acetic acid during the polymerization stage in conjunction with mechanical obstruction apparatus minimizes escape of monomer reactants in the polymerization step and produces a more satisfactory color product.